In a semiconductor manufacturing factory, Flon has been widely used in order to cool down wafers or the likes. However, in the recent years, there is a tendency to use liquid nitrogen as a cold source in place of Flon, because of the danger of causing an environmental destruction by Flon.
In the case where liquid nitrogen is used as a cold source, there is no need of a refrigerating system comprising a compressor, a condenser and etc. In general, it is designed that after liquid nitrogen is introduced from a storage tank into a factory's building through a main pipe, it is supplied to various kinds of semiconductor manufacturing units in clean rooms or other equipments (for example, a cold trap of a high vacuum apparatus and a cooling apparatus for a general heating source) through branch pipes extending from the main pipe.
For the main pipe and branch pipes in such a piping installation for liquid nitrogen, there is used a vacuum insulated pipe to keep cold. From the viewpoints of keeping the cold of liquid nitrogen or preventing its evaporation, in the prior art, such a vacuum insulated pipe includes a thick heat insulating layer or a plurality of heat insulating layers provided in a high vacuum space, and as a result, the heat insulating layer and outer pipe thereof become larger in diameter. Thus, a vacuum insulated pipe whose outer diameter is 60 mm or larger has been used for not only the main pipe but also the branch pipes.
In a piping installation in which vacuum insulated pipes of the large outer diameter are used, however, there is such a problem that a space to be provided with the pipes can not be effectively utilized because the occupation rate of the pipes in the space is high. In particular, it is not desirable to install branch pipes of the large outer diameter in a clean room, because the production cost of the clean room per unit volume is highly expensive.
If the outer diameter of the branch pipes is large, liquid nitrogen will be supplied in an amount larger than the amount of its practical use in a semiconductor manufacturing unit or the like, and as a result, most of liquid nitrogen supplied to the semiconductor manufacturing unit or the like will be discharged as it keeps the utilizable cold. In the prior art, this discharged cold medium is merely released to the atmosphere after its temperature is lowered to normal temperature by a discharge treatment system, and this results in failure in satisfactory thermal utilization of liquid nitrogen.